We are planning to issue 100 SAN with a single certificate. Would there be any performance issue ? If yes, what would the implications be ? What highest no# of SAN would be seamlessly supported with a single certificate ?

That depends on your server, but I think probably not (if there’s only one cert existing in your system) and vHost numbers are low…(again, this totally depend on your server hardwares and optimizations.)

kantharia:

What highest no# of SAN would be seamlessly supported with a single certificate ?

Let’s encrypt allows you to get maximum of 100 SANs per certificate (that could be mixed wildcards and single domains)